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        <div id="book">
        <div>
        <h1>The Node Beginner Book</h1>

        <div id="author">A Node.js tutorial by <a href="http://twitter.com/manuelkiessling">Manuel Kiessling</a></div>

        <a name="about"></a>

        <h2>About</h2>

        <p>
            The aim of this document is to get you started with developing
            applications with Node.js, teaching you everything you need to
            know about "advanced" JavaScript along the way. It goes way
            beyond your typical "Hello World" tutorial.
        </p>

        <a name="status"></a>

        <h3>Status</h3>

        <p>
            You are reading the final version of this book, i.e., updates are only
            done to correct errors or to reflect changes in new versions of Node.js.
            It was last updated on August 18, 2016.
        </p>

        <p>
            The code samples in this book are tested to work with both the
            Long Term Support version 4.5.0 as well as the most current 6.4.0 version
            of Node.js.
        </p>

        <p>
            This site allows you to read the first 19 pages of this book
            for free.
            The complete text is available as a DRM-free eBook (PDF, ePub and
            Kindle format). More info is available <a href="#salespitch">at the
            end of the free part</a>.
        </p>

        <a name="intended-audience"></a>

        <h3>Intended audience</h3>

        <p>
            This document will probably fit best for readers that have a
            background similar to my own: experienced with at least one
            object-oriented language like Ruby, Python, PHP or Java, only little
            experience with JavaScript, and completely new to Node.js.
        </p>

        <p>
            Aiming at developers that already have experience with other
            programming languages means that this document won't cover
            really basic stuff like data types, variables, control structures
            and the likes. You already need to know about these to understand
            this document.
        </p>

        <p>
            However, because functions and objects in JavaScript are different
            from their counterparts in most other languages, these will be
            explained in more detail.
        </p>

        <a name="structure"></a>

        <h3>Structure of this document</h3>

        <p>
            Upon finishing this document, you will have created a complete web
            application which allows the users of this application to view web
            pages and upload files.
        </p>

        <p>
            Which, of course, is not exactly
            world-changing, but we will go some extra miles and not only create
            the code that is "just enough" to make these use cases possible,
            but create a simple, yet complete framework to cleanly separate the
            different aspects of our application. You will see what I mean in a
            minute.
        </p>

        <p>
            We will start with looking at how JavaScript development in Node.js
            is different from JavaScript development in a browser.
        </p>

        <p>
            Next, we will stay with the good old tradition of writing a "Hello
            World" application, which is a most basic Node.js application that
            "does" something.
        </p>

        <p>
            Then, we will discuss what kind of "real" application we want to
            build, dissect the different parts which need to be implemented to
            assemble this application, and start working on each of these parts
            step-by-step.
        </p>

        <p>
            As promised, along the way we will learn about some of the more
            advanced concepts of JavaScript, how to make use of them, and
            look at why it makes sense to use these concepts instead of
            those we know from other programming languages.
        </p>

        <p>
            The source code of the finished application is available through
            <a href="https://github.com/ManuelKiessling/NodeBeginnerBook/tree/master/code/application">the
                NodeBeginnerBook Github repository</a>.
        </p>

        <div id="table-of-contents-headline">Table of contents</div>
        <div id="table-of-contents">
            <ul>

                <li><a href="#about">About</a>
                    <ul>
                        <li><a href="#status">Status</a></li>
                        <li><a href="#intended-audience">Intended audience</a></li>
                        <li><a href="#structure">Structure of this document</a></li>
                    </ul>
                </li>

                <li><a href="#javascript-and-nodejs">JavaScript and Node.js</a>
                    <ul>
                        <li><a href="#javascript-and-you">JavaScript and You</a></li>
                        <li><a href="#a-word-of-warning">A word of warning</a></li>
                        <li><a href="#server-side-javascript">Server-side JavaScript</a></li>
                        <li><a href="#hello-world">"Hello World"</a></li>

                    </ul>
                </li>
                <li><a href="#a-full-blown-web-application-with-nodejs">A full blown web application with Node.js</a>
                    <ul>
                        <li><a href="#the-use-cases">The use cases</a></li>
                        <li><a href="#the-application-stack">The application stack</a></li>
                    </ul>

                </li>
                <li><a href="#building-the-application-stack">Building the application stack</a>
                    <ul>
                        <li><a href="#a-basic-http-server">A basic HTTP server</a></li>
                        <li><a href="#analyzing-our-http-server">Analyzing our HTTP server</a></li>
                        <li><a href="#passing-functions-around">Passing functions around</a></li>
                        <li><a href="#how-function-passing-makes-our-http-server-work">How function passing makes our
                            HTTP server work</a></li>

                        <li><br/><strong>Chapters available in the full book:</strong><br/><br/>

                        <li>Event-driven asynchronous callbacks</li>
                        <li>How our server handles requests</li>
                        <li>Finding a place for our server module</li>
                        <li>What's needed to "route" requests?</li>
                        <li>Execution in the kingdom of verbs</li>
                        <li>Routing to real request handlers</li>

                            <li>Making the request handlers respond
                                <ul>
                                    <li>How to not do it</li>
                                    <li>Blocking and non-blocking</li>
                                    <li>Responding request handlers with non-blocking operation</li>
                                </ul>
                            </li>
                            <li>Serving something useful
                                <ul>
                                    <li>Handling POST requests</li>
                                    <li>Handling file uploads</li>
                                </ul>
                            </li>
                            <li>Conclusion and outlook</li>
                        </li>
                    </ul>
                </li>
            </ul>
        </div>

        <a name="javascript-and-nodejs"></a>

        <h2>JavaScript and Node.js</h2>

        <a name="javascript-and-you"></a>

        <h3>JavaScript and You</h3>

        <p>
            Before we talk about all the technical stuff, let's take a
            moment and talk about you and your relationship with
            JavaScript. This chapter is here to allow you to estimate
            if reading this document any further makes sense for you.
        </p>

        <p>
            If you are like me, you started with HTML "development"
            long ago, by writing HTML documents. You came across this
            funny thing called JavaScript, but you only used it in a
            very basic way, adding interactivity to your web pages
            every now and then.
        </p>

        <p>
            What you really wanted was "the real thing", you wanted to
            know how to build complex web sites - you learned a
            programming language like PHP, Ruby, Java, and started
            writing "backend" code.
        </p>

        <p>
            Nevertheless, you kept an eye on JavaScript, you saw that
            with the introduction of jQuery, Prototype and the likes,
            things got more advanced in JavaScript land, and that this
            language really was about more than <em>window.open()</em>.
        </p>

        <p>
            However, this was all still frontend stuff, and although it
            was nice to have jQuery at your disposal whenever you felt
            like spicing up a web page, at the end of the day you were,
            at best, a JavaScript <em>user</em>, but not a JavaScript
            <em>developer</em>.
        </p>

        <p>
            And then came Node.js. JavaScript on the server, how cool
            is that?
        </p>

        <p>
            You decided that it's about time to check out the old, new
            JavaScript. But wait, writing Node.js applications is
            one thing; understanding why they need to be written the
            way they are written means - understanding JavaScript.
            And this time for real.
        </p>

        <p>
            Here is the problem: Because JavaScript really lives two,
            maybe even three lives (the funny little DHTML helper from
            the mid-90's, the more serious frontend stuff like jQuery
            and the likes, and now server-side), it's not that easy to
            find information that helps you to learn JavaScript the
            "right" way, in order to write Node.js applications in a
            fashion that makes you feel you are not just using
            JavaScript, you are actually developing it.
        </p>

        <p>
            Because that's the catch: you already are an experienced
            developer, you don't want to learn a new technique by just
            hacking around and mis-using it; you want to be sure that
            you are approaching it from the right angle.
        </p>

        <p>
            There is, of course, excellent documentation out there.
            But documentation alone sometimes isn't enough. What is
            needed is guidance.
        </p>

        <p>
            My goal is to provide a guide for you.
        </p>

        <a name="a-word-of-warning"></a>

        <h3>A word of warning</h3>

        <p>
            There are some really excellent JavaScript people out
            there. I'm not one of them.
        </p>

        <p>
            I'm really just the guy I talked about in the previous
            paragraph. I know a thing or two about developing backend
            web applications, but I'm still new to "real" JavaScript
            and still new to Node.js. I learned some of the more
            advanced aspects of JavaScript just recently.
            I'm not experienced.
        </p>

        <p>
            Which is why this is no "from novice to expert" book. It's
            more like "from novice to advanced novice".
        </p>

        <p>
            If I don't fail, then this will be the kind of
            document I wish I had when starting with Node.js.
        </p>

        <a name="server-side-javascript"></a>

        <h3>Server-side JavaScript</h3>

        <p>
            The first incarnations of JavaScript lived in browsers.
            But this is just the context. It defines what you can
            do with the language, but it doesn't say much about what
            the language itself can do. JavaScript is a "complete"
            language: you can use it in many contexts and achieve
            everything with it you can achieve with any other
            "complete" language.
        </p>

        <p>
            Node.js really is just another context: it allows you to run
            JavaScript code in the backend, outside a browser.
        </p>

        <p>
            In order to execute the JavaScript you intend to run in the
            backend, it needs to be interpreted and, well, executed.
            This is what Node.js does, by making use of Google's V8 VM, the
            same runtime environment for JavaScript that Google
            Chrome uses.
        </p>

        <p>
            Plus, Node.js ships with a lot of useful modules, so you don't
            have to write everything from scratch, like for example
            something that outputs a string on the console.
        </p>

        <p>
            Thus, Node.js is really two things: a runtime environment and a
            library.
        </p>

        <p>
            In order to make use of these, you need to install Node.js.
            Instead of repeating the process here, I kindly ask you to
            visit
            <a href="https://nodejs.org/en/download/" title="Building and Installing Node.js">the
                official
                installation page</a>. Please come back once you
            are up and running.
        </p>

        <a name="hello-world"></a>

        <h3>"Hello World"</h3>

        <p>
            Ok, let's just jump in the cold water and write our first
            Node.js application: "Hello World".
        </p>

        <p>
            Open your favorite editor and create a file called
            <em>helloworld.js</em>. We want it to write "Hello World"
            to STDOUT, and here is the code needed to do that:
        </p>
        <pre class="prettyprint lang-js"><span class="pln">console</span><span class="pun">.</span><span
                class="pln">log</span><span class="pun">(</span><span class="str">"Hello World"</span><span class="pun">);</span></pre>
        <p>
            Save the file, and execute it through Node.js:
        </p>

        <pre>node helloworld.js</pre>
        <p>
            This should output <em>Hello World</em> on your terminal.
        </p>

        <p>
            Ok, this stuff is boring, right? Let's write some real
            stuff.
        </p>


        <a name="a-full-blown-web-application-with-nodejs"></a>

        <h2>A full blown web application with Node.js</h2>

        <a name="the-use-cases"></a>

        <h3>The use cases</h3>

        <p>
            Let's keep it simple, but realistic:
        </p>

        <div>
          <ul>
              <li>
                  The user should be able to use our application with
                  a web browser
              </li>
              <li>
                  The user should see a welcome page when
                  requesting http://<em>domain</em>/start which displays a
                  file upload form
              </li>
              <li>
                  By choosing an image file to upload and submitting the
                  form, this image should then be uploaded to
                  http://<em>domain</em>/upload, where it is displayed once
                  the upload is finished
              </li>
          </ul>
        </div>

        <p>
            Fair enough. Now, you could achieve this goal by googling
            and hacking together <em>something</em>. But that's not
            what we want to do here.
        </p>

        <p>
            Furthermore, we don't want to write only the most basic
            code to achieve the goal, however elegant and correct this code
            might be. We will intentionally add more abstraction than
            necessary in order to get a feeling for building more
            complex Node.js applications.
        </p>

        <a name="the-application-stack"></a>

        <h3>The application stack</h3>

        <p>
            Let's dissect our application. Which parts need to be
            implemented in order to fulfill the use cases?
        </p>

        <div>
          <ul>
              <li>
                  We want to serve web pages, therefore we need an
                  <strong>HTTP server</strong>
              </li>
  
              <li>
                  Our server will need to answer differently to
                  requests, depending on which URL the request was
                  asking for, thus we need some kind of
                  <strong>router</strong> in order to map requests
                  to request handlers
              </li>
              <li>
                  To fulfill the requests that arrived at the server
                  and have been routed using the router, we need
                  actual <strong>request handlers</strong>
              </li>
              <li>
  
                  The router probably should also treat any incoming
                  POST data and give it to the request handlers in
                  a convenient form, thus we need <strong>request
                  data handling</strong>
              </li>
              <li>
                  We not only want to handle requests for URLs, we
                  also want to display content when these URLs are
                  requested, which means we need some kind of
                  <strong>view logic</strong> the request handlers
                  can use in order to send content to the user's
                  browser
              </li>
              <li>
                  Last but not least, the user will be able to upload
                  images, so we are going to need some kind of
                  <strong>upload handling</strong> which takes care of
                  the details
              </li>
          </ul>
        </div>

        <p>
            Let's think a moment about how we would build this stack
            with PHP. It's not exactly a secret that the typical setup
            would be an Apache HTTP server with mod_php5 installed.
            <br>
            Which in turn means that the whole "we need to be able to
            serve web pages and receive HTTP requests" stuff doesn't
            happen within PHP itself.
        </p>

        <p>
            Well, with node, things are a bit different. Because with
            Node.js, we not only implement our application, we also
            implement the whole HTTP server. In fact, our web
            application and its web server are basically the same.
        </p>

        <p>
            This might sound like a lot of work, but we will see in a
            moment that with Node.js, it's not.
        </p>

        <p>
            Let's just start at the beginning and implement the first
            part of our stack, the HTTP server.
        </p>

        <a name="building-the-application-stack"></a>

        <h2>Building the application stack</h2>

        <a name="a-basic-http-server"></a>

        <h3>A basic HTTP server</h3>

        <p>
            When I arrived at the point where I wanted to start with my
            first "real" Node.js application, I wondered not only how to
            actually code it, but also how to organize my code.
            <br>
            Do I need to have everything in one file? Most tutorials on
            the web that teach you how to write a basic HTTP server in
            Node.js have all the logic in one place. What if I want to
            make sure that my code stays readable the more stuff I
            implement?
        </p>

        <p>
            Turns out, it's relatively easy to keep the different
            concerns of your code separated, by putting them in
            modules.
        </p>

        <p>
            This allows you to have a clean main file, which you
            execute with Node.js, and clean modules that can be used by
            the main file and among each other.
        </p>

        <p>
            So, let's create a main file which we use to start our
            application, and a module file where our HTTP server code
            lives.
        </p>

        <p>
            My impression is that it's more or less a standard to name
            your main file <em>index.js</em>. It makes sense to put our
            server module into a file named <em>server.js</em>.
        </p>

        <p>
            Let's start with the server module. Create the file
            <em>server.js</em> in the root directory of your project,
            and fill it with the following code:
        </p>
        <pre class="prettyprint lang-js"><span class="kwd">var</span><span class="pln"> http </span><span
                class="pun">=</span><span class="pln"> require</span><span class="pun">(</span><span
                class="str">"http"</span><span class="pun">);</span><span class="pln"><br><br>http</span><span
                class="pun">.</span><span class="pln">createServer</span><span class="pun">(</span><span class="kwd">function</span><span
                class="pun">(</span><span class="pln">request</span><span class="pun">,</span><span class="pln"> response</span><span
                class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln"><br>&nbsp; response</span><span
                class="pun">.</span><span class="pln">writeHead</span><span class="pun">(</span><span
                class="lit">200</span><span class="pun">,</span><span class="pln"> </span><span
                class="pun">{</span><span class="str">"Content-Type"</span><span class="pun">:</span><span
                class="pln"> </span><span class="str">"text/plain"</span><span class="pun">});</span><span
                class="pln"><br>&nbsp; response</span><span class="pun">.</span><span class="pln">write</span><span
                class="pun">(</span><span class="str">"Hello World"</span><span class="pun">);</span><span
                class="pln"><br>&nbsp; response</span><span class="pun">.</span><span class="pln">end</span><span
                class="pun">();</span><span class="pln"><br></span><span class="pun">}).</span><span
                class="pln">listen</span><span class="pun">(</span><span class="lit">8888</span><span
                class="pun">);</span></pre>

        <p>
            That's it! You just wrote a working HTTP server. Let's
            prove it by running and testing it. First, execute your
            script with Node.js:
        </p>
        <pre>node server.js</pre>
        <p>
            Now, open your browser and point it at
            <a href="http://localhost:8888/" rel="nofollow">http://localhost:8888/</a>.
            This should display a web page that says "Hello World".
        </p>

        <p>
            That's quite interesting, isn't it. How about talking about
            what's going on here and leaving the question of how to
            organize our project for later? I promise we'll get back to
            it.
        </p>

        <a name="analyzing-our-http-server"></a>

        <h3>Analyzing our HTTP server</h3>

        <p>
            Well, then, let's analyze what's actually going on here.
        </p>

        <p>
            The first line <em>require</em>s the <em>http</em> module
            that ships with Node.js and makes it accessible through the
            variable <em>http</em>.
        </p>

        <p>
            We then call one of the functions the http module offers:
            <em>createServer</em>. This function returns an object, and
            this object has a method named <em>listen</em>, and takes
            a numeric value which indicates the port number our HTTP
            server is going to listen on.
        </p>

        <p>
            Please ignore for a second the function definition that
            follows the opening bracket of <em>http.createServer</em>.
        </p>

        <p>
            We could have written the code that starts our server and
            makes it listen at port 8888 like this:
        </p>
        <pre class="prettyprint lang-js"><span class="kwd">var</span><span class="pln"> http </span><span
                class="pun">=</span><span class="pln"> require</span><span class="pun">(</span><span
                class="str">"http"</span><span class="pun">);</span><span class="pln"><br><br></span><span class="kwd">var</span><span
                class="pln"> server </span><span class="pun">=</span><span class="pln"> http</span><span
                class="pun">.</span><span class="pln">createServer</span><span class="pun">();</span><span
                class="pln"><br>server</span><span class="pun">.</span><span class="pln">listen</span><span class="pun">(</span><span
                class="lit">8888</span><span class="pun">);</span></pre>

        <p>
            That would start an HTTP server listening at port 8888
            and doing nothing else (not even answering any incoming
            requests).
        </p>

        <p>
            The really interesting (and, if your background is a more
            conservative language like PHP, odd looking) part is the
            function definition right there where you would expect the
            first parameter of the <em>createServer()</em> call.
        </p>

        <p>
            Turns out, this function definition IS the first (and only)
            parameter we are giving to the <em>createServer()</em>

            call. Because in JavaScript, functions can be passed around
            like any other value.
        </p>

        <a name="passing-functions-around"></a>

        <h3>Passing functions around</h3>

        <p>
            You can, for example, do something like this:
        </p>
        <pre class="prettyprint lang-js"><span class="kwd">function</span><span class="pln"> say</span><span
                class="pun">(</span><span class="pln">word</span><span class="pun">)</span><span
                class="pln"> </span><span class="pun">{</span><span class="pln"><br>&nbsp; console</span><span
                class="pun">.</span><span class="pln">log</span><span class="pun">(</span><span
                class="pln">word</span><span class="pun">);</span><span class="pln"><br></span><span
                class="pun">}</span><span class="pln"><br><br></span><span class="kwd">function</span><span class="pln"> execute</span><span
                class="pun">(</span><span class="pln">someFunction</span><span class="pun">,</span><span class="pln"> value</span><span
                class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln"><br>&nbsp; someFunction</span><span
                class="pun">(</span><span class="pln">value</span><span class="pun">);</span><span
                class="pln"><br></span><span class="pun">}</span><span class="pln"><br><br>execute</span><span
                class="pun">(</span><span class="pln">say</span><span class="pun">,</span><span
                class="pln"> </span><span class="str">"Hello"</span><span class="pun">);</span></pre>

        <p>
            Read this carefully! We pass the
            function <em>say</em> as the first parameter to the
            <em>execute</em> function. Not the return value of
            <em>say</em>, but <em>say</em> itself!
        </p>

        <p>
            Thus, <em>say</em> becomes the local variable
            <em>someFunction</em> within <em>execute</em>, and execute
            can call the function in this variable by issuing
            <em>someFunction()</em> (adding brackets).
        </p>

        <p>
            Of course, because <em>say</em> takes one parameter,
            <em>execute</em> can pass such a parameter when calling
            <em>someFunction</em>.
        </p>

        <p>
            We can, as we just did, pass a function as a parameter to
            another function by its name. But we don't have to take this
            indirection of first defining, then passing it - we can
            define and pass a function as a parameter to another
            function in-place:
        </p>
        <pre class="prettyprint lang-js"><span class="kwd">function</span><span class="pln"> execute</span><span
                class="pun">(</span><span class="pln">someFunction</span><span class="pun">,</span><span class="pln"> value</span><span
                class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln"><br>&nbsp; someFunction</span><span
                class="pun">(</span><span class="pln">value</span><span class="pun">);</span><span
                class="pln"><br></span><span class="pun">}</span><span class="pln"><br><br>execute</span><span
                class="pun">(</span><span class="kwd">function</span><span class="pun">(</span><span
                class="pln">word</span><span class="pun">){</span><span class="pln"> console</span><span
                class="pun">.</span><span class="pln">log</span><span class="pun">(</span><span
                class="pln">word</span><span class="pun">)</span><span class="pln"> </span><span
                class="pun">},</span><span class="pln"> </span><span class="str">"Hello"</span><span
                class="pun">);</span></pre>

        <p>
            We define the function we want to pass to <em>execute</em>
            right there at the place where <em>execute</em> expects its
            first parameter.
        </p>

        <p>
            This way, we don't even need to give the function a name,
            which is why this is called an <em>anonymous function</em>.
        </p>

        <p>
            This is a first glimpse at what I like to call "advanced"
            JavaScript, but let's take it step by step. For now, let's
            just accept that in JavaScript, we can pass a function as
            a parameter when calling another function. We can do this
            by assigning our function to a variable, which we then
            pass, or by defining the function to pass in-place.
        </p>

        <a name="how-function-passing-makes-our-http-server-work"></a>

        <h3>How function passing makes our HTTP server work</h3>

        <p>
            With this knowledge, let's get back to our minimalistic
            HTTP server:
        </p>
        <pre class="prettyprint lang-js"><span class="kwd">var</span><span class="pln"> http </span><span
                class="pun">=</span><span class="pln"> require</span><span class="pun">(</span><span
                class="str">"http"</span><span class="pun">);</span><span class="pln"><br><br>http</span><span
                class="pun">.</span><span class="pln">createServer</span><span class="pun">(</span><span class="kwd">function</span><span
                class="pun">(</span><span class="pln">request</span><span class="pun">,</span><span class="pln"> response</span><span
                class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln"><br>&nbsp; response</span><span
                class="pun">.</span><span class="pln">writeHead</span><span class="pun">(</span><span
                class="lit">200</span><span class="pun">,</span><span class="pln"> </span><span
                class="pun">{</span><span class="str">"Content-Type"</span><span class="pun">:</span><span
                class="pln"> </span><span class="str">"text/plain"</span><span class="pun">});</span><span
                class="pln"><br>&nbsp; response</span><span class="pun">.</span><span class="pln">write</span><span
                class="pun">(</span><span class="str">"Hello World"</span><span class="pun">);</span><span
                class="pln"><br>&nbsp; response</span><span class="pun">.</span><span class="pln">end</span><span
                class="pun">();</span><span class="pln"><br></span><span class="pun">}).</span><span
                class="pln">listen</span><span class="pun">(</span><span class="lit">8888</span><span
                class="pun">);</span></pre>

        <p>
            By now it should be clear what we are actually doing here:
            we pass the <em>createServer</em> function an anonymous
            function.
        </p>

        <p>
            We could achieve the same by refactoring our code to:
        </p>
        <pre class="prettyprint lang-js"><span class="kwd">var</span><span class="pln"> http </span><span
                class="pun">=</span><span class="pln"> require</span><span class="pun">(</span><span
                class="str">"http"</span><span class="pun">);</span><span class="pln"><br><br></span><span class="kwd">function</span><span
                class="pln"> onRequest</span><span class="pun">(</span><span class="pln">request</span><span
                class="pun">,</span><span class="pln"> response</span><span class="pun">)</span><span
                class="pln"> </span><span class="pun">{</span><span class="pln"><br>&nbsp; response</span><span
                class="pun">.</span><span class="pln">writeHead</span><span class="pun">(</span><span
                class="lit">200</span><span class="pun">,</span><span class="pln"> </span><span
                class="pun">{</span><span class="str">"Content-Type"</span><span class="pun">:</span><span
                class="pln"> </span><span class="str">"text/plain"</span><span class="pun">});</span><span
                class="pln"><br>&nbsp; response</span><span class="pun">.</span><span class="pln">write</span><span
                class="pun">(</span><span class="str">"Hello World"</span><span class="pun">);</span><span
                class="pln"><br>&nbsp; response</span><span class="pun">.</span><span class="pln">end</span><span
                class="pun">();</span><span class="pln"><br></span><span class="pun">}</span><span class="pln"><br><br>http</span><span
                class="pun">.</span><span class="pln">createServer</span><span class="pun">(</span><span class="pln">onRequest</span><span
                class="pun">).</span><span class="pln">listen</span><span class="pun">(</span><span
                class="lit">8888</span><span class="pun">);</span></pre>

        <p>
            Maybe now is a good moment to ask: Why are we doing it
            that way?
        </p>

        <a name="event-driven-callbacks"></a>

        <h3>Event-driven asynchronous callbacks</h3>

        <a name="continuereading"></a>
        <p>
            To understand why Node.js applications have to be written this way,
            we need to understand how Node.js executes our code. Node's
            approach isn't unique, but the underlying execution model is
            different from runtime environments like Python, Ruby, PHP or Java.
        </p>
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